Transformers equipped with tap changers are frequently used in different types of power transmission environments, such as at 10 kV and above. A transformer that comprises a tap changer is able to change the turns ratio between the windings to thereby change voltage levels. This ability is in many systems used for controlling the delivery of power.
Transformers are generally reliable. The probability of them failing is low, such as around 1%. However, of those 1% that fail, typically 20-40% are due to failure in the tap changer.
The reason for this is that the tap changer is the only part of the transformer that has mechanically moving elements. Therefore this part of the transformer is more likely to cause a failure than the rest of the transformer.
It is therefore of interest to protect a transformer equipped with a tap changer through monitoring the tap changer.
The protection is intended to detect if the tap changer operation is carried out properly or not, because an improper tap changer operation may have catastrophic consequences. Therefore, the protection also has to be fast.
The present invention is directed towards such transformer protection.
EP 2541 572 discloses a protection arrangement for a tap changer where the current through the tap changer is detected and used to generate a current indication signal. The duration of the current indication signals is then compared with a threshold, and a fault indication signal generated if it is.
WO02/48730 discloses a protection arrangement for a tap changer, where condition diagnosing of the tap changer is made based on the actual temperature, expected heat exchange between the tap changer and ambient air and transformer, amount of heat generated by the tap changer and actual fluid temperature.
US 2007/0225945 disclosed a protection arrangement for a tap changer. The temperature of the tap changer after tap change is compared with a first threshold that is related to the tap changer temperature before the tap change operation.